In the manufacture of many magnetic digital disc and drum memory systems, it is necessary to record a closed clock track around the disc or drum. Such a closed clock track consists of a certain number of uniformly spaced magnetic flux reversals, so that as the disc or drum rotates a continuous train of clock pulses may be read out from the magnetic read head associated with the clock track. It is important in these systems for the train of pulses to have a high degree of uniformity, and for the point of closure where the end of the writing of the flux reversals meets the beginning to be indistinguishable from the other parts of the clock track. Systems for writing closed clock tracks around a magnetic memory disc or drum of acceptable quality are known in the art. For example, U.S. Patent 3,540,022 which issued November 10, 1970 to the present inventor describes one such system.
One object of the present invention is to provide an improved system for writing a closed clock track around a magnetic memory disc or drum which will permit clock magnetic flux reversals to be written at higher frequency and higher density than was possible in the prior art systems. A second object is to provide an improved system for writing auxiliary tracks, commonly called sector or address tracks, around a magnetic memory, disc or drum simultaneously with the writing of the clock track. U.S. Patent 3,755,790 which issued August 28, 1973 to the present inventor describes one type of prior art apparatus for writing sector and address tracks on a rotating magnetic memory disc or drum.
It is usual in the prior art to record a clock track on a rotating magnetic memory drum or disc in the following manner. First, an index, or origin, pulse is written on a first track on the magnetic memory disc or drum. Then, by means of an oscillator whose frequency can be controlled either manually or automatically, a series of uniformly spaced magnetic flux reversals are recorded on a second, temporary intermediate clock track on the magnetic memory. Whether manual or automatic, the procedure is to start writing the oscillator output onto the second memory track when an index pulse is received, and stop writing the oscillator output when the next index pulse is received. The number of clock pulses so written on the second track is then counted (or determined at the time of writing). If the number is too low, the oscillator frequency is increased and the operation is repeated; and if the number is too high, the oscillator frequency is decreased, and another attempt is made. The aforesaid operations are repeated until the exact desired number of pulses has been written on the temporary clock track. The temporary track now has the number of flux reversals desired for the final clock track. Such a method is described, for example, in Patent 3,540,022, and such a method may be used to synchronize the clock oscillator used in the system of the present invention.
The temporary track in the prior art systems has some degree of what is commonly called closure error. Closure error is the failure of the flux reversals at the beginning and end of the temporary clock track to match exactly, so that the spacing between the beginning flux reversal and the final flux reversal is not the same as, and is not indistinguishable from, the spacing of all the other flux reversals in the temporary track.
In addition to closure error, the flux reversals in the temporary clock track in the prior art system may be amplitude modulated at the point of closure, this being due to the effect of turning off the output from the clock oscillator too abruptly.
The final steps in the prior art procedure, accordingly, are to read the flux reversals on the temporary clock track, process the resulting clock pulses through a phase-locked loop, and to write the output of the phase-locked loop onto a third final clock memory track on the rotating memory disc or drum. During these final steps the amplitude of the output of the clock oscillator is reduced very slowly to zero and over a period of time corresponding to many memory revolutions, so as to avoid amplitude modulations of the flux reversals in the final clock track.
The phase-locked loop in the prior art system includes a filter which has a narrow band-pass, so that the closure error of the clock pulses derived from the temporary track on the rotating memory may be filtered out, and so that the closure error may appear to a much lesser degree in the final clock track.
The prior art procedure described in the preceding paragraphs has certain disadvantages. For example, the density and frequency of the flux reversals in the final clock track, which are the same as the density and frequency of the flux reversals in the temporary clock track must be limited in such prior art systems to a value which does not seriously reduce the amplitude of the flux reversals in the temporary clock track, and the ability of the system to read the flux reversals in the temporary clock track in the presence of read/write crosstalk. The foregoing criterion limits the density and frequency of the flux reversals in the final clock track to that which can be read in the presence of writing during the clock track recording process, rather than the higher density and frequency which can be read in the absence of writing during normal operation of the memory.
A second disadvantage in the prior art system is that frequently the system cannot sense some of the flux reversals in the temporary track because of the amplitude modulation thereof due to the write turn-off operation, thereby causing the control of the turn-off to be unnecessarily critical.
A third disadvantage in the prior art system is that because the temporary clock track has no intentional position mark encoded in it, the temporary clock track cannot conveniently be used to record final sector or address tracks simultaneously with the final clock track. This is because such sector and address tracks frequently require a position reference to establish their alignment.
It is accordingly, a further object of the present invention to provide a system for writing the flux reversals in the temporary clock track and for reading the flux reversals in the temporary clock track, such that the disadvantages of the prior art system described above are obviated.
Flux reversals in the temporary clock track in the system of the invention are written at a lower density and frequency than in the final clock track. In the preferred embodiment of the invention, the flux reversals in the temporary clock track are written at one-eleventh of the frequency of the flux reversals in the final clock track. Therefore, for each eleven flux-reversal pairs in the final clock track, one flux-reversal pair is written in the temporary clock track. At the beginning of the temporary clock track, however, one flux-reversal pair is written for each twelve flux-reversal pairs in the final clock track, and this occurs no fewer than one time, and no more than eleven times, as required to accomodate the remainder when the certain number of flux-reversal pairs in the final clock track is divided by eleven.
For example, if the number of flux-reversal pairs in the final clock track is 11,003 there will be one flux-reversal pair in the temporary clock track corresponding to the first twelve flux-reversal pairs in the final clock track; one flux-reversal pair corresponding to the second twelve flux-reversal pairs of the final clock track; one flux-reversal pair corresponding to the third twelve flux-reversal pairs of the final clock track; and then one flux-reversal pair for each eleven flux-reversal pairs in the final clock track from there on to the end. The total number of flux-reversal pairs in the temporary clock track is therefore one thousand. It can be seen that this improved temporary clock track overcomes the aforementioned disadvantages of the prior technique, since the density and frequency of the flux reversals in the final clock track are not limited by the flux reversals in the temporary clock track because the flux reversals in the temporary clock track are written at much lower density and frequency. Moreover, the flux-reversals in the temporary clock track in the system of the invention are written so far apart in time and distance that the output from the write oscillator may be turned off entirely between flux-reversals, thus obviating amplitude modulation of the flux-reversals due to decaying current at the time of writing turn-off. The first occurrence of the more widely spaced flux-reversals in the temporary clock track (where they are spaced twelve units apart instead of eleven) serves as a position marker in the temporary clock track which may be used to establish alignment of sector and/or address tracks.